<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
     <style>
    div{ width:50px; height: 50px; border:1px solid;}
</style>
</head>
<body>
     <div></div>
     <p> <button id="btn">change</button> </p>
     <script>
   
    var btn = document.getElementById('btn');
    var div = document.getElementsByTagName('div')[0];
    btn.onclick = function () {
        div.style.border = 'double red';
    }
     </script>
</body>
</html>